Our efforts in the coming year will be directed in part towards improving the procedures of isolating pluripotent hemopoietic stem cells and T lymphocyte progenitors for rat bone marrow using the fluorescence-activated cell sorter. We will also initiate developmental studies to demonstrate the existence of a common stem cell for T and b lymphocytes as well as a discrete B cell progenitor. Other studies will concern the functional properties and developmental interrelationships of Thy-l positive and Thy-l negative members of the B and T cell series in the rat, with emphasis on the stages of immunological diversification, immunological commitment, and tolerization.